Switched Reluctance Machine is a term used for both Switch Reluctance Motors and Switch Reluctance Generators. The abbreviation SR stands for Switched Reluctance. The abbreviation SRM is used for SR Motors or SR Machines, the latter including SR Generators.
The principle of operation of SR machines is well known in the art, although SR machines are much less used as compared to Brushless DC (BLDC) motors for example. A good reference document to SR machines is “Switched Reluctance Motors And Their Control” by T. J. E. Miller. Many references can be found on the operation of a switched reluctance motor. The document by T. J. E. Miller also shows that many variants exist in number of teeth, phases, topology and control. The number of teeth can vary considerably, from 2 stator teeth and 2 rotor teeth to high numbers (for example 8 or 12 or more). An example of an SR machine with 12 stator teeth and 8 rotor teeth is shown in FIG. 1. Around the stator teeth a winding is wound (not explicitly shown but indicated with letter A and A′ for the first winding, B and B′ for the second winding, etc.), one or more of these windings constitute the phase winding of the switched reluctance machine.
SR machines are typically driven by asymmetric H-bridges (see FIG. 5) to control the current in the phase windings. These windings can be assembled in 2, 3, 4, 5 or more phases. An example with three-phase windings A, B, C is shown in FIG. 2.
FIG. 4 shows an ideal flux-linkage (Ψ) versus current (I) diagram, known in the art, representing the energy conversion from electrical energy into mechanical in an SR motor, as the rotor moves from an unaligned position to an aligned position and to the next unaligned position. Starting from a first working point wp1 with no current in the phase winding, a current Ic is applied (by an electrical drive circuit, not shown), and the working point quickly moves to working point wp2, the rotor still being in an unaligned position, but now a current Ic is flowing through the phase winding. Assuming that the current Ic is maintained for a while, the rotor moves from an unaligned position (wp2) to an aligned position (wp3). Then the current in the phase winding quickly decreases to zero, and the working point moves quickly from wp3 to wp4, which in the flux-linkage versus current diagram coincides with working point wp1. The area of the shape described by the working points represents the energy W converted from electrical to mechanical.
Apart from the functional aspects described above (mainly related to controlling the power switches as a function of angular rotor position), a proper design of a drive circuit should also take into account the power rating of the components. It is well known that components with a higher power rating are typically also more expensive, and in case of Switched Reluctance Motors/Generators, the power electronics can form an important fraction of the total cost (e.g. up to 50%). Hence large efforts are made in attempts to reduce the system cost by lowering the cost of the power electronics. The use of a lower number of power switches or the use of more common (in the sense of: more readily available, less exotic) power electronics are two possibilities to reduce the cost.
FIG. 6 shows the usual converter topology for a brushless DC motor control and for induction motors. These full transistor bridges got so common that 6 transistors with internal freewheel diode got much cheaper than 6 transistors with external diodes in asymmetric H-bridges of FIG. 5. Unfortunately, these full transistor bridges cannot be used to drive SR machines, unless only one transistor of each full bridge would be used, which is very cost ineffective. Referring back to FIG. 4, the converted energy is represented by the area W defined by the working points wp1-wp4, which has rounded corners in practice. The maximal peak-peak flux is maximally Vd×T/2, where Vd is the supply voltage of the full bridge and T is the electrical period of the voltage profiles over, or the current profiles in the phase winding. The converted energy W is typically 60% of the peak to peak flux×Imax. The maximal converted energy per period for one phase is typically about 0.6×Vd×Imax/2, hence the average power is typically about 0.3×Vd×Imax. In a three-phase converter for a brushless DC motor (see e.g. FIG. 6), the maximal converted average power for one phase (two transistors) is (⅔)×V1/2*Imax=0.33×Vd×Imax, where Vd is the supply voltage, and Imax is the current flowing through the windings A, B, C. Note that the voltage over one phase A, B, C is about equal to half the voltage Vd/2 of the series connection of the two transistors. If the same calculation is done for a synchronous machine under sine wave current and the circuit of FIG. 6, the line to line voltages is limited to the DC bus, the star voltage to Vd/√3 and the maximal average power gets Vd/√3×Imax/2*cos(φ)=0.289×Vd×Imax×cos(φ), where φ is the phase difference between the first harmonic of the current and first harmonic of the voltage.
Referring back to drive circuits for SR machines, in particular to the drive circuit of FIG. 2, due to the external diodes, the switched reluctance drive circuits need two times more legs for a similar power rating, which makes the hardware more expensive.
U.S. Pat. No. 5,923,142 describes a reluctance machine system including a reluctance machine having a rotor and a stator and one additional auxiliary field winding. The publication “A Magnetless Axial-Flux Machine for Range-Extended Electric Vehicles” by Lee in Energies 2014, 7(3), 1483-1499; doi:10.3390/en7031483 describes a reluctance machine system including a reluctance machine having a rotor and a stator and two additional auxiliary field windings. Constant current(s) are established in the auxiliary field winding(s) to assist in the establishment of a magnetic field in the machine to improve the machine's performance, or stated in other words: to provide a motor with more power without having to increase the power rating of the switches, but at the expense of changing the physical SR motor by adding an additional winding and means for powering it. A good understanding of this document may help to better understand the present invention.
There is still room for improvements or alternatives.